1. Field of the Invention
This invention relates to a method for producing semiconductors, and more specifically, this invention relates to an all-vapor method for producing p-type tellurium-containing II-VI Semiconductor and Ohmic Contacts thereof.
2. Background of the Invention
Semiconductors are made from foundation material having electrical conductivity intermediate to metals and insulators. This foundation material primarily consists of crystalline material (for example silicon and germanium) having a small number of free electrons which have escaped from their respective atoms. The electron-lacking atoms possess vacancies called holes which are also able to move throughout the crystalline material.
Conductivity of the crystalline lattice can be modified with dopants. For example, some dopants (e.g., arsenic, nitrogen, phosphorous, bismuth) release free electrons, thereby making the lattice more conductive. Such a more conductive lattice is called an n-type semiconductor and has more electrons than holes.
Other dopants (e.g. aluminum, boron, gallium, indium) scavenge free electrons, thereby making the lattice less conductive, i.e. more insulative. Such a less conductive lattice is called a p-type semiconductor.
II-VI semiconductors (whereby the II-VI designate Group II-VI elements of the periodic table) are nonlinear optical materials which experience a change in their electrical charge distribution when exposed to light. As such, these materials have potential in solar cell manufacture and related areas such as optical switching. Further, existing art has demonstrated numerous methods for depositing thin polycrystalline films of these semiconductors.
The fabrication of solar cells incorporating tellurium-containing II-VI semiconductors (e.g. CdS/CdTe materials) is known to increase conversion efficiency but involves processing that is potentially costly with respect to hazardous reagents and waste products. This is partly because the fabrication of solar cells produces secondary waste streams during acid rinsing and etching steps required in their conventional manufacture. Customary fabrication of these II-VI type cells involves the aforementioned acid rinsing and etching steps.
Existing technologies for fabricating CdS/CdTe thin-film solar cells (see, for example U.S. Pat. Nos. 4,456,630 and 4,873,198) rely on wet chemistry steps to accomplish various chemical and electronic modifications of the CdS/CdTe layers and the Cd/Te surface. For example, increased grain size and doping are achieved by thermally activated treatment of the CdS/CdTe structure with CdCl.sub.2, which is typically applied from liquid solution. Any excess CdCl.sub.2 is then rinsed from the structure as waste or else captured for recycling.
After this chlorine treatment, it is still necessary to modify the CdS/CdTe structure before ohmic contact can be made. This is accomplished by etching with, for example, nitric acid or hydrazine, all of which must be collected and disposed of as waste. In the specific case of these reagents, their innate properties are not conducive to use in a safe, low-cost manufacturing facility.
Another problem inherent with the conventional fabrication of these tellurium-containing II-VI semiconductors is the susceptibility to spatial variability and irreproducibility depending on the uniformity of the CdCl.sub.2 layer. Acid rinsing and etching create non-uniformity of the CdCl.sub.2 layer itself and non-uniformity between successive fabrications of these CdCl.sub.2 layers.
Another obstacle in the conventional technology is that in light of the aforementioned waste problems, fabrication can be difficult to manage over large module areas, up to 20 square feet per module. Module fabrication processes would have to contend with the application, recovery and recycling of the CdCl.sub.2 containing solutions.
A need exists in the art for a method in which CdCl.sub.2 chemical species are uniformly reacted with the CdTe surface. A need also exists for a method in which deposition of a CdCl.sub.2 layer can be eliminated from the processing. Following the reaction with CdCl.sub.2, a need exists for forming a highly conductive p-type surface on the CdTe to facilitate forming ohmic contact. Lastly, a need exists in the art to facilitate large module fabrication.